1. Field of the Invention
The present invention relates generally to a dot line printer, and more particularly to a dot line printer for printing L dot lines at a time during each of forward and backward movements of a hammer bank accommodating therein a plurality of print hammers juxtaposed along a print line, and for minimizing the number of print hammers simultaneously fired, where L is an integer equal to or greater than 3.
2. Description of the Prior Art
In a dot line printer, a print hammer which is a plate-like leaf spring having a print pin at its tip end is held in a non-printing position by a magnetic attraction of a permanent magnet against elastic deformation of the print hammer. The print hammer is released from the non-printing position with a supply of a release current to an electromagnetic coil. The print pin thus impinges upon a sheet of print paper, thereby making a dot impression thereon.
As disclosed in Japanese Patent Publication (Kokoku) No. 55-10385, there is a problem such that if adjacent print hammers are allowed to be fired at the same time, duration of the current supplied to the electromagnetic coil needs to be prolonged or the peak level of the current needs to be increased. To obviate such a problem, this Publication proposes displacing columnar printing types embossed on the periphery of a drum in the circumferential direction with respect to printing types in adjacent column so that adjacent print hammers are not fired at the same time.
Another proposal for solving this problem is disclosed in Japanese Patent Publication (Kokai) No. 58-11177 and Japanese Patent Publication (Kokoku) No. 62-28755, wherein a plurality of print hammers are divided into two groups and respective ones of the print hammers in a second group are displaced from their home positions by a distance corresponding to one half of a reciprocal of a print density (hereinafter referred to as "a half dot") where the print density is represented by a number of printable dots per a unit length. Displacement of the second group print hammers by the half dot reduces electric power consumption and lowers the peak level of the current flowing in the electromagnetic coil. As a consequence, a small-capacity power source can be used for the dot line printer.
However, high speed dot line printers have an increased number of print hammers, say nearly 300, so that the number of simultaneously fired print hammers increases in due course even if the hammers are divided into two groups, thus electrical and mechanical influences caused by the increased number of the hammers are still outstanding in the high speed dot line printers. More specifically, as the number of simultaneously fired print hammers is increased, a total amount of currents flowing in the coils is increased, thus requiring a large-capacity power source. Further, mechanical impact increases as the larger number of print hammers impinge upon a platen at a time. This increased mechanical impact may cause the print gap between the print hammers and the sheet of paper to widen, thus lowering print quality. Magnetic interaction between adjacent hammers is another problem in such high speed printers, because the pitch between the hammers is made small so as to accommodate the increased number of print hammers.